In electromechanical systems requiring fast response and four quadrant operation with good performance near zero speed, an electrical machine must essentially provide controlled torque over a wide range of operating conditions. Historically, a separately excited direct current (DC) motor has been the primary machine type employed in such situations. The proportional relationship between motor armature current and motor torque provides a direct means of achieving torque control. Use of high frequency DC choppers with current feedback provides direct control of current and overcomes the problem of speed dependence caused by the armature circuit counter emf. Excellent torque control can be achieved until the counter emf becomes comparable to the chopper input voltage. Field weakening is also available to allow operation in the high speed, constant HP range.
Developments in the theory of controlling alternating current (AC) machines coupled with technological developments in power switching systems and control electronics now provide capability for achieving controlled torque operation of AC machines. As in the DC machine, torque control is obtained by controlling the motor current. In the AC machine, however, this control must be in terms of both amplitude and phase, which has led to the generic term "vector control". In addition, unlike the DC machine where orientation of field flux and armature mmf is fixed by the commutator and brushes, the AC machine requires external control of the field flux and armature mmf spatial orientation. Without such control, phase angles between the various fields in an AC machine vary with load (and during transients) giving rise to complex interactions and oscillatory dynamic response. Control systems for AC machines which directly control these space angles have come to be called "field orientation" or "phase angle" or "vector" controllers. The term "field orientation" generally means a system which attempts to produce a 90.degree. space angle between specifically chosen field components (currents and/or fluxes) so as to closely emulate a DC machine.
High frequency chopping and current feedback are used to obtain current control and overcome the speed dependent counter emf in field orientation control of an AC machine. A pulse width modulated (PWM) inverter with current loop control has been the controller of choice, although voltage control is feasible and other types of inverters are often used.
Notwithstanding the improvement in operating characteristics of the AC machine using such phase angle or field orientation controllers, the desire to provide a machine having the advantageous characteristics of the DC machine has led to development of a brushless DC machine and, more particularly, to a permanent magnet DC machine, hereinafter referred to as an electronically commutated motor or ECM. In the ECM, there are provided multiple field windings which are energizable in a selectable sequence to establish a rotating magnetic field. A rotor, constructed of permanent magnets, has a substantially constant magnetic flux orientation which interacts with the rotating magnetic flux field of the field windings to effect rotation of the rotor. A more detailed description of an ECM may be had by reference to U.S. Pat. No. 4,005,347 to Erdman issued Jan. 25, 1977 and assigned to the assignee of the present application.
Control systems developed for the ECM have generally been PWM inverter systems using either square-wave current or voltage control to regulate motor torque. These systems typically require rotor position sensors to inform the control electronics of the instantaneous rotor position to insure proper energization patterns for the stator windings to produce rotation. Such rotor position sensors (e.g., encoders, resolvers, Hall effect devices) are undesirable because of their cost, volume and susceptibility to damage and failure. Alternatively, it is possible to eliminate the need for these sensors by measuring the back emf voltages generated by the spinning rotor magnets to determine rotor position. An exemplary control system of this latter type is disclosed in U.S. Pat. No. 4,654,566 to Erdman, issued Mar. 31, 1987 and assigned to the assignee of the present invention.
While the ECM using the aforementioned PWM control techniques has resulted in an electromechanical system which combines many of the characteristic advantages of the DC machine with the low maintenance and high speed capability of the AC machine, it is desirable to achieve similar performance characteristics in other types of permanent magnet machines which are supplied with sinusoidal voltage or current excitation instead of square-wave excitation.
FIG. 2 is a perspective view of one form of ECM or permanent magnet (PM) motor M with multiple stator windings. As illustrated, the rotor is removed from the stator. An illustrative three-phase winding for motor M is shown in FIG. 3A. Rotor 15 is constructed with alternating magnetic polarity magnets shown in what is sometimes referred to as a surface-magnet construction. While this construction is very simple and well known, it is also possible to construct such rotors in what is known as "interior magnet" form to create an interior permanent magnet (IPM) motor. As shown in FIG. 3, the magnets in the IPM motor are mounted below the rotor surface and overlaid with a magnetic material which serves to protect the magnets and strengthen the rotor. While this provides an improved structure, it creates special control problems in that the magnetic material is susceptible to induced magnetic flux fields from the motor stator winding currents. Furthermore, the IPM motor requires sinusoidal excitation for smooth torque production. Thus operation of this type of sinusoidally-excited PM machine without a rotor position sensor produces special control problems since the back-EMF cannot be directly detected at the motor terminals as in a square-wave ECM drive due to the inducted magnetic flux from the stator current causing an addition to the back EMF.
The sinusoidally-excited permanent magnet motor also demonstrates a start-up problem not experienced in field orientation control of AC induction motors if there is no position sensor. In an AC induction motor, the magnetic flux in the rotor can be initially established in an arbitrary orientation by excitation of the stator windings with a current of predetermined orientation. The rotor of the induction motor is symmetrical about its axis so that there is no preferred rotor excitation at startup. In the permanent magnet motor, the permanent magnets establish a fixed orientation for the rotor field excitation flux. The applied stator current must be properly oriented with respect to the rotor magnets to generate torque. This poses a problem since orientation of the magnets is not known at startup. Without some means of determining the rotor orientation, precise torque control at start-up cannot be achieved in the absence of a rotor position sensor.